Slight double vision when driving tired late at night. what is that?

Mild muscle weakness: Most common explanation would be a slight misalignment of the eyes when tired. This kind of double vision should go away when you close either eye. Other possible causes would not go away when one eye is closed. A type of glasses lens called prism may be able to help this symptom. You should be seen by an eye md for diagnosis and treatment recommendations.

Double vision: Could be due to being near sighted or having some astigmatism that becomes more symptomatic at night when the pupil size dilates.